The use of flexible disk media, sometimes known as "floppy disks", for storage of magnetically recorded information is very well known. In the most common form of floppy disk file, the disk is generally enclosed in a jacket, to form a combination sometimes known as a "diskette". To record or read information, a magnetic transducing head is brought into contact with the disk medium through an access slot in the jacket and the disk is rotated by a drive mechanism at linear velocities around 3.5 meters per second.
Although cheap and reliable, the wear and performance limitations of this technology have always been recognised and several attempts have been made to develop a flexible disk system in which the disk would ride on an air bearing out of contact with both the head and any support or enclosure. This would lower wear and permit higher operating speeds. One very early step in this direction is described in an article by R. T. Pearson entitled "The Development of a Flexible Disk Magnetic Recorder" (Proc IRE, Vol. 49, No. 1, Jan. 1961, pp 164-174). In this article, it is observed that when a flexible disk is rotated in proximity to a rigid fixed plate, and ventilation is provided at the disk hub, then the disk moves toward the plate and adopts a stable form flying at a separation from the plate that decreases toward the outside diameter. The case of flat, grooved and perforated backing plates are analysed. In the described apparatus, the backing plate is part of a fixed drive mechanism and the head approaches the disk from the side opposite to the plate.
The mechanism of this type of device is notionally simple in that the disk forms a vaneless impeller accelerating the air between disk and plate tangentially. For large initial separation there is a reduction of pressure in the trapped volume according to the centrifugal pump mechanism, which moves the disk closer to the plate. Local stability is reached as the radial component of viscous drag increases with the narrowing gap and balances the local pressure gradient due to centripetal acceleration. The total flow of air and hence the gap at any radius is determined principally by the axial position of the hub with respect to the plate and the stiffness of the disk near the hub for disk thicknesses of interest in this field, the local separation is dominated by local viscous drag and is highly stable. Flow, for most configurations, is entirely laminar.
The ideal laminar flow which can be achieved over a flat continuous plate is not so easily maintained in those file arrangements where a slot is provided in the plate to permit access by the head. Both the head and the slot perturb the airflow leading to unstable head to disk spacing. This has led to the development of various non-planar plate configurations.
One such arrangement is shown in UK Pat. No. 1,542,568 where a sealed head access slot in a backing plate is bordered on both sides by smoothly contoured ridges. It is possible for the slot to be sealed because the backing plate forms part of the fixed drive mechanism and is not removable with the disk.
Another configuration of backing plate is shown in UK Pat. No. 1,525,308. In this patent, a disk cartridge is described which includes a backing plate within the cartridge. The backing plate is provided with an unsealed slot to permit head access. Entry of additional air by way of the unsealed slot is a further cause of instability. In order to overcome this the plate is bent about two chords on opposite sides of the access slot. Such a configuration gives the rotating disk greater positional stability and, thus, better quality signals from the head.
Although cartridges for the disks are clearly necessary to prevent contamination if they are to be interchanged between files, the provision of a backing plate as part of the enclosure leads to an expensive and bulky cartridge. However, the presence of a precision machined backing plate has generally proved necessary if a stable head to disk separation is to be achieved. One compromise, avoiding the expense of a machined plate in each cartridge, is shown in U.S. Pat. No. 4,400,748 in which the precision backing plate is made part of the fixed drive mechanism of the disk file and enters the cartridge only when it is loaded into the drive.
Two proposals for a flexible disk cartridge in which the disk flies directly over the inside wall of a cartridge are also known. In U.S. Pat. No. 4,159,494, a cartridge is shown which has opposite chordal bends so that the disk flies close to one wall and then the other, permitting head access from both sides. In U.S. Pat. No. 4,279,623, a cartridge is shown in which the backing surface is provided by one inner wall of the cartridge. The wall has a head access slot located in a convex raised portion of its surface. The opposite wall of the cartridge is provided with bumps to urge the disk towards the head access surface.
In the type of disk file described above, employing a backing surface over which the disk flies, the design of the head surface is also an important factor in achieving close and stable separation between head and media. The actual head transducing surface is generally part spherical in shape and may be slotted. U.S. Pat. No. 4,003,091 describes how the provision of a polished stabilising ring sealed around the head proper tends to stabilise the head/disk separation. The use of a similar stabilised head in conjunction with a backing plate is also shown in European patent application, publication No. 0064390-A1.